A broadband terahertz generation method by means of difference frequency terahertz wave generation using the Cherenkov radiation has conventionally been proposed (refer to FIG. 2 of Non-Patent Document 1). Exciting light from a light source (KTP-OPO excited by Nd-YAG laser, for example) outputting two wavelengths is made incident to a nonlinear crystal (MgO-doped LN (lithium niobate) crystal, for example) according to FIG. 2 of Non-Patent Document 1. The incident exciting light having the two wavelengths induces nonlinear polarization in the MgO-doped LN crystal. If the MgO-doped LN crystal satisfies the condition (nTHz>nopt) of the Cherenkov radiation, a spherical wave which takes the maximal value at each position corresponding to the twice of the coherence length is generated. This spherical wave has a wave front aligned in a direction of a radiation angle θ satisfying the following relationship, and the terahertz waves are emitted in this direction. It should be noted that nopt denotes a refractive index in the exciting light waveband of the MgO-doped LN crystal, and nTH, is a refractive index in the terahertz waveband of the MgO-doped LN crystal.cos θ=nopt/nTHz 
On this occasion, the MgO-doped LN crystal is formed on an LN substrate which is not doped. The terahertz wave (Cherenkov light) generated in the MgO-doped LN crystal is a spherical wave, and is radiated extending in a conical shape. Thus, terahertz waves departing from the non-doped LN substrate, and a terahertz wave proceeding toward the non-doped LN substrate exist in the terahertz waves generated in the MgO-doped LN crystal.    (Non-Patent Document 1) K. Suizu, K. Koketsu, T. Shibuya, T. Tsutsui, T. Akiba, and K. Kawase, “Extremely frequency-widened terahertz wave generation using Cherenkov-type radiation,” Opt. Express 17(8), 6676-6681 pages, 2009